Refrigeration-cycle equipment having a compressor, a radiator, a pressure reducer, an evaporator have been used in the past in an air conditioner, a car air conditioner, an electric refrigerator (freezer), cold or refrigerated warehouse, a showcase and the like. Such refrigeration-cycle equipment have used as the refrigerant hydrocarbons containing fluorine atoms.
In particular, since hydrocarbons containing both fluorine atoms and chlorine atoms (HCFC, hydrochlorofluorocarbons) have high performance, and are incombustible and nontoxic to humans, they have been widely used in refrigeration-cycle equipment.
However, it has been known that since HCFCs (hydrochlorofluorocarbons) contain chlorine atoms, when they are released in the air and reach the stratosphere, they destroy ozone layers HFCs (hydrofluorocarbons), which do not contain chlorine atoms are being used in place of HCFCs, and do not destroy ozone layers. HFC's, however, have a large greenhouse effect because they have a long life in the air, and cannot be said to be a satisfactory refrigerant for preventing undesirable global warming.
The feasibility of refrigeration-cycle equipment using CO2 is being studied. The ozone depletion potential (ODP) of CO2 is zero, and its global warming factor is markedly small compared to halogen-atom-containing hydrocarbons, such as HCFCs and HFCs, which contain halogen atoms. For example, refrigeration-cycle equipment using CO2 is proposed in Japanese Patent Publication No. 7-18602.
This Japanese Patent Publication discloses that the critical temperature of CO2 is 31.1° C. and the critical pressure is 7,372 kPa, and the refrigeration-cycle equipment using CO2, can operate in a transcritical cycle described using FIG. 4.
FIG. 4 is a Mollier diagram of a refrigeration cycle using CO2 as a refrigerant.
As A-B-C-D-A in the drawing shows, by the compression stroke (A-B) for compressing CO2 refrigerant in a gas-phase state with a compressor, the cooling stroke (B-C) for cooling the high-temperature high-pressure CO2 refrigerant in a super critical state with a radiator (gas cooler), the pressure-reducing stroke (C-D) for reducing the pressure with a pressure reducer, and the evaporation stroke (D-A) of the evaporator for evaporating the CO2 refrigerant in a gas-liquid two-phase state, heat is absorbed from an external fluid, such as the air, with the latent heat of evaporation, and the external fluid is cooled.
In FIG. 4, transition from the saturated vapor region (gas-liquid two-phase region) to the heated vapor region (gas-phase region) in the evaporation stroke (D-A) is performed in the same manner as in the case of HCFCs or HFCs, and the line (B-C) is located in the high-pressure side above the critical point CC and never intersects the saturated-liquid line and the saturated-vapor line.
Specifically, in the region exceeding the critical point CC (supercritical region), no condensation stroke as in the case of HCFCs or HFCs is present, but the cooling stroke wherein the CO2 refrigerant is cooled without being reliquefied.
At this time, since the working pressure of the refrigeration-cycle equipment using a CO2 refrigerant is about 3.5 MPa for the low-pressure-side pressure, and about 10 MPa for the high-pressure-side pressure, the working pressure is higher than in the case of using HCFCs or HFCs, and the high-pressure-side pressure and the low-pressure-side pressure are about 5 to 10 times the working pressure of the refrigeration-cycle equipment using HCFCs or HFCs.
The working pressure of the refrigeration-cycle equipment operating in the transient critical high pressure depends on several factors, such as the quantity of the filled refrigerant, the factor volume and the cooling stroke temperature, and if the working pressure deviates from the optimal high-pressure-side pressure during operation, relatively low freezing capacity and a low efficiency may result. Therefore, it is necessary to make the high-pressure-side pressure in operation agree to the optimal high-pressure-side pressure by controlling the quantity of the filled refrigerant during the operation of the refrigeration-cycle equipment at rest, to achieve a relatively high freezing capacity and a high efficiency.
To achieve this, Japanese Patent No. 2804844 proposes that the volume of the high-pressure-side circuit should be large relative to the volume of the low-pressure-side circuit, and more specifically, it proposes that the volume of the high-pressure-side circuit should be 70% or more of the total internal volume, and that the refrigerant quantity of the filled CO2 refrigerant should be 0.55 to 0.70 kg per liter on the basis of the total internal volume. The entire disclosure of the reference of Japanese Patent No. 2804844 is incorporated herein by reference in its entirety.
However, in order that the refrigerant flow path of the heat exchanger used in the radiator or the evaporator of such refrigeration-cycle equipment resists the pressure of the high-pressure refrigerant, a flat tube 51 having a plurality of through-holes 51a of a small bore diameter is shown in the schematic diagram of FIG. 5.
In order to minimize the pressure loss of the refrigerant in the heat exchanger or connecting pipes, it is desirable to enlarge the sectional area of the low-pressure-side refrigerant circuit, rather than the sectional area of the high-pressure-side refrigerant circuit.
Furthermore, in order to resist the pressure of the high-pressure refrigerant, it is desirable that the shell of the compressor is of a low-pressure shell type. As a result, the volume of the low-pressure-side circuit including the shell space of the compressor becomes relatively larger than the volume of the high-pressure-side circuit.
Specifically, the volume of the high-pressure-side circuit normally becomes less than 70% the total internal volume. Here, the high-pressure-side circuit means the component elements and connecting pipes (specifically, the discharging portion of the compressor, the radiator, the pressure reducer and the like) wherein the CO2 refrigerant of relatively high pressure operates during the operation of the refrigeration-cycle equipment, among the closed circuit constituting the refrigeration-cycle equipment. The low-pressure-side circuit means the component elements and connecting pipes wherein the CO2 refrigerant of relatively low pressure operates (specifically, the pressure reducer, the evaporator, the compressor and the like).
In refrigeration-cycle equipment wherein the volume of the high-pressure-side circuit is less than 70% the total internal volume, when the quantity of the filled CO2 refrigerant is large, or the quantity of the oil discharged together with the CO2 refrigerant is large, there is the possibility of the rapid pressure rise in the high-pressure-side circuit.
The rapid pressure rise occurs due to the fact that the density of the CO2 refrigerant in the high-pressure-side circuit increases when the quantity of the refrigerant retained in the low-pressure-side circuit is transferred to the high-pressure-side circuit of a relatively small volume; or that the oil discharged together with the CO2 refrigerant further decreases the volume of the high-pressure-side circuit of a relatively small volume. This occurs easily especially in the startup of the refrigeration-cycle equipment. When the rapid pressure rise occurs in the high-pressure-side circuit, problems may arise, such that the high-pressure protecting mechanism operates to stop the compressor in order to protect the radiator, the evaporator and the compressor of the refrigeration-cycle equipment from the high pressure, and thereby startup becomes difficult.